Typically, liquid crystal display (LCD) devices are used as flat panel display devices. LCD devices are popular as they have high picture quality and low power consumption.
The LCD device controls optical transmittance by injecting liquid crystal between two substrates and by controlling an intensity of an electric field applied between electrodes on one or both of the substrates. The LCD device is widely used in notebook computers, personal digital assistants (PDA), video telephones, televisions, various portable electronic devices, etc.
In recent years, LCD devices with various characteristics have been developed due to consumer demands. These characteristics include increased screen size, finer pitch, and wide viewing angle.
Various wide viewing angle LCD devices have been developed. One such LCD device uses a horizontal field driving method, while another LCD device includes a compensated film driven by a vertical alignment (VA) method.
The LCD device using a horizontal field driving method has a pixel electrode and a common electrode formed on the same substrate. In this case, an electric field horizontal to the substrate is formed between the pixel electrode and the common electrode when the pixel electrode is turned on. Accordingly, a long axis of liquid crystal molecules is operated in parallel with the substrate according to the on/off state of the pixel electrode. As a result, a refractive index of the liquid crystal observed by the user's naked eyes is small, thereby the LCD device has an excellent contrast ratio and wide viewing angle.
On the other hand, the LCD device using a vertical alignment (VA) method implements a viewing angle by compensation. In this LCD device, a compensation film is attached at an inner side of a polarizing plate. The LCD device using the vertical alignment (VA) method comprises a pair of transparent substrates having a transparent electrode at an inner surface thereof, a liquid crystal material injected between the two substrates and aligned to be perpendicular to the substrate, and a pair of polarizing plates perpendicularly attached to an outer surface of each transparent substrate for polarizing light.
Recently, a LCD device using a different mode has been proposed. In this device, the liquid crystal molecules are divided by forming an opening pattern or a protrusion on a transparent electrode without performing rubbing.
A general LCD device for implementing a viewing angle will be explained with reference to FIG. 1.
FIG. 1 is a sectional view schematically showing a liquid crystal display (LCD) device in accordance with the conventional art.
As shown, a general LCD device comprises a thin film transistor (TFT) array substrate 11, a color filter substrate 41 facing and attached to the TFT array substrate 11 with a uniform gap therebetween, and a liquid crystal layer 51 formed between the TFT array substrate 11 and the color filter substrate 41.
On the TFT array substrate 11, pixels are arranged in a matrix. A thin film transistor 20, a pixel electrode 27, and a capacitor (not shown) are formed at each unit pixel. The thin film transistor 20 comprises a gate electrode 13 formed on the TFT array substrate 11, a gate insulating layer 15 formed on the TFT array substrate 11 including the gate electrode 13, a semiconductor layer 17, and source/drain electrodes 21 and 23 formed on the semiconductor layer 17 and spaced from each other with a certain gap therebetween. The pixel electrode 27 is electrically connected to the drain electrode 23 through a drain contact hole (not shown) formed in a passivation layer 25 formed on an entire surface of the TFT array substrate 11 including the source/drain electrodes 21 and 23.
On the color filter substrate 41, a common electrode 47 for applying an electric field to the liquid crystal layer 51 with the pixel electrode 27, R, G, and B color filters 45 for implementing colors, and a black matrix 43 are formed.
An alignment layer (not shown) is formed at each rear surface of the TFT array substrate 11 and the color filter substrate 41. Also, liquid crystal is arranged on a surface of the alignment layer in a certain direction by rubbing.
The liquid crystal is rotated by dielectric anisotropy when an electric field is applied between the pixel electrode 27 formed at each pixel of the TFT array substrate 11 and the common electrode 47 formed at the front surface of the color filter substrate 41. Accordingly, images are displayed by passing light through each pixel or by preventing light from being transmitted through each pixel.
The color filter substrate 41 and the TFT array substrate 11 are respectively provided with a first polarizing plate 61 and a second polarizing plate 63. The polarizing plates 61 and 63 polarize natural light incident with vibration in several directions into one direction (that is, polarization).
In the LCD device, light from a backlight is emitted to a front surface of an LCD panel, and the light passes through several diffusion sheets (not shown) in order to obtain a uniform brightness. As the light is incident on the front surface of the LCD panel, an image can be displayed in a wide viewing angle.
The general LCD device has a wide viewing angle. However, in specific cases, the LCD device can be fabricated so as to have a narrow viewing angle. Thus, even if an image can be displayed in a wide viewing angle, since light is incident on the front surface of the LCD panel, changing between a narrow viewing angle and a wide viewing angle cannot be actively controlled.
Although not shown, a viewing angle can be narrowed by applying a film-type filter or an optical fiber to the LCD panel. However, a display mode cannot be switched between a wide viewing angle and a narrow viewing angle. Furthermore, using such techniques results in a drastic decrease in the brightness of the front surface of the LCD panel.